The present invention relates to a method of soldering an electronic component to a substrate by transient liquid phase soldering, wherein a solder paste is deposited and melted on the substrate, the electronic component is placed in the melted solder by a handling tool and the melted solder is allowed to solidify.
When soldering electronic components to a substrate a problem arises in the field of optoelectronics or other applications in which the components need to be placed very accurate on the substrate. For example, the optical center of light emitting surfaces of LEDs have to be positioned very accurate with respect to reference features of the substrate or board to enable cost effective and accurate assembly of a head lamp, i.e. assembly of the primary and secondary optics. In many applications PCB-boards (PCB: printed circuit board) are used as the substrate to which the electronic components have to be soldered. The tolerances of the PCB industry are large, typically in the order of 100 μm, in comparison with optoelectronic requirements in the range of 10 μm. The tolerance chain, e.g. reference features to pads, reflow soldering, top to bottom feature tolerance of the carrier, add to over 100 μm.
Transient liquid phase (TLP) solder pastes based on standard SAC paste (SAC: SnAgCu) are currently under development to form high reliable bonds for high temperature applications. TLP soldering in general has the advantage that a solder joint is formed at a low temperature but remelts at a significant higher temperature, i.e. the solder solidifies during the solder process. For many optoelectronic applications, as for example automotive low and high beam modules, a very high post soldering accuracy is required, i.e. in the range of 10 μm. Standard reflow processes cannot reach such a high accuracy because the reflow process itself has a tolerance (standard deviation 5 to 10 μm) and in addition the tolerance between solder pads and reference structures on the boards are also in the order of 50 μm. Therefore the known process of self alignment of electronic components on a liquid solder surface does not allow the exact positioning of the electronic component.